1. Field of the Invention
The present invention relates to a rotary actuator for a magnetic disc device, and more particularly to a rotary actuator which is less affected by a leakage magnetic flux generated by a voice coil.
2. Description of the Prior Art
A magnetic disc device has an actuator to position a magnetic head onto a desired track of a disc which is an information record medium for recording and reproducing information. Usually, the actuator supports a plurality of arms having the magnetic heads at one end of a head carriage and drives the head carriage radially of the disc or circumferentially of the disc. The head carriage is driven by a voice coil motor which comprises a moving coil which protrudes from the other end of the head carriage and a magnetic circuit assembly including a permanent magnet and a yoke.
FIG. 1 shows a main portion of a prior art voice coil motor. A magnetic circuit comprises a vertical yoke 7 and horizontal yokes 5A, 5B and 6 which are assembled in an E-shape, and permanent magnets 2A and 2B attached to the inner surfaces of the upper and lower horizontal yokes 5A and 5B, respectively. Gaps 3A and 3B are formed between the center horizontal yoke 6 and the permanent magnets 2A and 2B, respectively, and a moving coil 20 which drives the head carriage (not shown) surrounds the center yoke 6 and is moved horizontally along the gaps 3A and 3B.
In the vicinity of the moving coil, leakage magnetic fluxes 10 from the exposed sides 4A and 4B of the permanent magnets 2A and 2B cannot be neglected but they affect the magnetic heads at the end of the actuator. FIG. 2 shows a distribution of the leakage magnetic fluxes of the magnetic circuit assembly. As shown by arrows G.sub.1 and G.sub.2 on the right hand, the leakage magnetic fluxes are dominant in the vicinities of contact areas 200A and 200B of the permanent magnets 2A and 2B and the upper and lower horizontal yokes 5A and 5B, respectively. A reason therefor may be that the leakage magnetic fluxes increase close to magnetic flux saturation densities of the yokes 5A and 5B in the vicinities of the contact areas 200A and 200B. The magnitude of the leakage magnetic fluxes increases as a thickness l.sub.2 of the permanent magnets 2A and 2B is increased relative to a length l.sub.1 of the gaps 3A and 3B. For a swing (rotary) type actuator, the leakage magnetic flux in the vicinity of the magnetic head amounts to approximately one gauss. The leakage magnetic fluxes act as a DC biasing magnetic field to an AC magnetic field when the information is read or written by the magnetic head and distort the AC magnetic field which inherently is symmetric. As a result, they degrade the recording and reproducing characteristics.
In order to reduce the influence of the leakage magnetic fluxes, the prior art magnetic disc device has a magnetic shield member arranged in the vicinity of the magnetic head to suppress the influence of the leakage magnetic fluxes from the voice coil motor as well as the magnetic influence from the external of the device. However, such a prior art construction is complex and cannot achieve a sufficient effect.